1. Field of the Invention
This invention relates to grease compositions and, specifically, to a grease designed for use in heavy duty, high temperature applications, such as for journal bearings on bits used to drill hot subterranean formations.
2. Description of the Prior Art
A variety of grease compositions have been employed in rock bits in the past. Such grease compositions typically comprise a high viscosity, refined petroleum or hydrocarbon oil which provides the basic lubricity of the composition and may constitute about 3/4 of the total grease composition as the oil is thickened with a metal soap or metal complex soap, wherein the metal is typically aluminum, barium, calcium, lithium, sodium or strontium. Complex-thickened greases are well known in the art and are discussed, for example, in Encyclopedia of Chemical Technology, Kirk-Othmer, 2nd Edition, A. Standen, Editor, Interscience Publishers, John Wiley & Sons, Inc., New York, N.Y., 1967, pages 582-587. It is also known to employ certain complexes, such as the calcium-acetate containing complexes and the lithium-hydroxy-stearate-containing complexes; to provide high temperature stability and maintain lubrication properties at the high temperatures to which the greases may be subjected.
The grease utilized to lubricate a rock bit of the type used to drill hot (frequently over 300.degree. F.) subterranean formations is subjected to severe and demanding constraints. The drilling takes place in an abrasive atmosphere of drilling mud and rock particles thousands of feet from the engineer or supervisor, who does not have benefit of oil pressure gauges or temperature sensors at the surfaces to be lubricated. The lubricant must possess properties which enable flow-through passageways to the surfaces to be lubricated and must prevent solid lubricant particles from settling out.
The prior art shows solid extreme pressure (EP) additives which have been employed to attempt to enhance the lubrication of properties of oils and greases. For example, molybdenum disulfide has been used in a wide variety of lubricants as discussed in U.S. Pat. Nos. 3,062,741; 3,170,878; 3,281,355; and 3,384,582 other solid additives include copper, lead and graphite.
It is also known to include metallic oxides like zinc oxide in lubrication oils. U S. Pat. No. 2,736,700 describes the use of molybdenum disulfide and a metallic oxide such as a fumed lead oxide and zinc oxide in a ratio of 2 parts molybdenum disulfide to 1 part metallic oxide, in a paint-on composition, or bonded lubricant, containing a lacquer drying agent. Such bonded lubricants are used for drawing tough metals, such as uranium, thorium, zinc and titanium. Such bonded lubricants are inadequate and could not be used in the low wear, heavily loaded applications for which this invention is intended.
U.S. Pat. No. 3,935,114 teaches the use of molybdenum disulfide and antimony trioxide in a lubricating grease for a journal bearing used in a drill bit. This grease has proved particularly effective when used in copper inlay-on-boronized bearings of rock bits.
The prior art also includes the use of fluorides of sodium, potassium and calcium as matrix materials in the surface layer of a dry bearing structure, particularly for aerospace applications. These fluorides were used in composite structures and were typically applied to the bearing surface by plasma spray, see e.g. U.S. Pat. No. 3,746,352. To our knowledge, calcium fluoride has not been used previously as a component of a grease to provide improved bearing performance particularly in rock bit bearings.
A need exists for a bearing grease with superior lubricating properties that can be employed in the application of lubricating journal bearings and bits drilling in an abrasive atmosphere.
A need also exists for a low wear grease which can be used with rock bit bearings to provide extended wear life and load carrying capacity.